To that end, a machine is customarily used comprising an electromagnetic wave generator, a chamber connected to the generator and made of a conductive material (generally metal), and an enclosure provided in the chamber and made of a material (generally quartz) that is transparent to the electromagnetic waves produced by the generator.
After insertion of the container (generally made of a thermoplastic polymer material such as PET) into the enclosure, they are both depressurized to establish a high vacuum in the container (several μbars, 1 μbar being equal to 10−6 bar) necessary to establish the plasma, and in the enclosure outside the container a medium vacuum (on the order of 30 mbar to 100 mbar) to prevent the container from contracting from the effect of the difference in pressure on either side of its wall.
A precursor gas (such as acetylene, C2H2) is then introduced into the container, said precursor being activated by electromagnetic bombardment (this generally involves low power UHF microwaves at 2.45 GHz) in order to cause it to go through the cold plasma state and thus generate species including hydrogenated carbon (including CH, CH2, CH3), which is deposited in a thin layer (whose thickness is customarily between 50 and 200 nm, depending on the case, 1 nm being equal to 10−9 m) on the inner wall of the container.
During the treatment, it is essential to avoid any communication between the interior and the exterior of the container. Otherwise, the positive pressure outside the container would cause the injection of air into it and the oxidation of the species generated by the plasma, to the detriment of the quality of the deposited barrier layer.
This is the reason known treatment machines are equipped with sealing devices comprising an annular gasket against which the opening (also called mouth) of the container is applied when it is inserted into the machine. By way of illustration, reference can be made to French patent application FR 2 872 148 (Sidel) or to the equivalent international application WO 2006/000539, or to the American patent U.S. Pat. No. 5,849,366.
This technique can be improved. Indeed, leaks have been observed at the sealing gasket. These leaks are due to the progressive depositing on the gasket of carbonaceous species generated by the plasma. Over time, this deposit forms on the surface of the gasket a film that decreases the flexibility thereof, and gaps appear at the interface between the gasket and the mouth of the container. In order to prevent the occurrence of such leaks, the solution currently being used is to clean or replace the gasket, which involves shutting down the machine for the duration of this maintenance work.